Objectives of this research proposal are to determine factors influencing luteinizing hormone release from bovine pituitaries in vivo and in vitro. We are investigating the role of gonadal steroids and gonadotropin releasing hormone (GnRH) in control of LH synthesis and release. Investigations designed to determine quantitative and qualitative changes in gonadal steroid output of bulls from birth to puberty are in progress. We plan to relate changes in gonadal hormone output with changes in pituitary responsiveness to GnRH in vivo and in vitro. In addition, experiments are proposed to determine the site of action of gonadal steroids in control of LH synthesis and release. In cows, our emphasis is to determine the mechanism by which the preovulatory surge of LH occurs and the nature of negative feedback inhibition on LH release. We are investigating the interaction of estrogen and progesterone in control of magnitude and time of occurrence of the preovulatory LH surge. In addition, we are measuring hormone changes in blood, hypothalamus and pituitary prior to and during the preovulatory LH surge in cows. Complementary studies will be carried out in vitro using bovine pituitary cells.